1. Field of the Invention
This invention is directed to the field of connectors and fasteners; in one particular aspect to a robot-friendly connector; and in another particular variety of environments, including but not limited to, under water and in extra vehicular activity in outer space.
2. Description of Related Art
In the past space structures and vehicles to be used in outer space have been launched as completely assembled and integrated systems. The size of proposed structures and vehicles has become prohibitively large to launch as complete assemblies. One alternative is to launch the structures in pieces and assemble them on orbit or on another celestial body. Structures which are assembled on location can usually be constructed out of lighter, more efficient sub-components because they do not have to withstand launch and landing loads in their as-assembled configurations. Assembly and maintenance of many of these structures will be accomplished by astronauts or by robotic devices.
Due to life support requirements and the numerous lengthy and monotonous steps to perform, human assembly of structures such as trusses in outer space is an inefficient endeavor. Extra-vehicular activity time for space station assembly missions is limited to 24 man-hours per flight, and there are numerous other more difficult and less monotonous tasks which would be better suited for an astronaut. A better approach delegates such repetitive and well-structured tasks to robotic devices. Structural assembly is repetitious, well structured, and labor intensive; it is therefore well suited for complete automation. "Robot-friendly" hardware for such assembly is very desirable. Such hardware incorporates many of the principles generally associated with design-for-assembly principles, e.g., simple insertion operations, generous lead-in chamfers, and self-aligning parts.
Various joint devices have been designed for building trusswork in outer space. These are generally in two categories: 1) hand-operated joints and 2) tool-assisted joints. With hand-operated joints truss members are assembled by extra vehicular activity ("EVA") astronauts wearing pressurized gloves. Tool assisted joints are assembled either by an EVA astronaut using a tool or by a robotic device using a special-purpose gripper.
Critical to the design of hand-operated fasteners is the ease with which a gloved astronaut can assemble hundreds of joints by hand without excessive fatigue or glove wear. These requirements have forced a compromise for simplicity of operation at the price of weight, complexity and cost. Each joint has its own internal actuating mechanism as well as an external mechanism (e.g., a lever or collar) which adapts the connector to the gloved astronaut's hand. Thus, the tool necessary to tighten the joint is part of each single unit. The designers of these joints have assumed that whatever motion an astronaut makes to operate this joint, a robot could be made to do as well. However, in order to duplicate these operations, the robot must perform extremely difficult motions and/or the gripper must incorporate an additional mechanism which allows it to mimic the motions of a gloved astronaut. This duplication results in a very complex overall system.
Tool-assisted fasteners requiring two-handed operation by gloved astronauts are cumbersome and virtually impossible to assemble with a single robot arm. Others have incorporated a mechanism including a tool which allows them to be operated with a simple motion (for an astronaut) such as a 90.degree. rotation of the tool. This motion is difficult for a robot arm and the design is unnecessarily complicated.
Another major disadvantage shared by prior designs is that they are very intolerant of large misalignments. Large insertion forces are required to fit together most of these joints with small linear or angular mismatch. Manufacturing tolerances or thermal distortion can easily cause excessive alignment error. A robot may introduce additional error due to gripper misalignment, deflection or compliance in the manipulator structure and joints, inaccuracies in the drive system, and inaccuracies in jigs which hold the truss during assembly. In terrestrial applications, parts of robots, e.g. arms, comply due to gravity so that a position indicated by a computer control may be inaccurate. Also, robots are machines which are subject to hysteresis, thermal expansion and friction. Prior art fasteners were not designed to accommodate such misalignments and are unsuitable for automated assembly.
Pending U.S. patent application Ser. No. 07/824,806 entitled "Quick Connect Fasteners for Assembling Devices in Space" filed on Mar. 15, 1992 discloses a robot-compatible joint for EVA use. The joint has threadedly-advanced expander rod which forces a set of collet fingers to expand outward into a receptacle to achieve connection. End effector drive engagement is accomplished by using a section of commercially available flexible drive shafting with an allen head wrench welded to the end. Before connection, the entire collet assembly is held inside the joint body by a light compression spring force. The joint halves are moved together with the assistance of a two-fingered external guide. Clocking (orientation of a joint about the strut axis) is performed by a wedge protruding from the joint face which engages a matching receptacle on the scar as the two halves are moved together. Then, the flex drive is inserted into a slot in the joint body. The top of the allen head wrench simply slides along the inside wall of the body, up a conical ramp, and into a matching hex hole in the end of the collet assembly. Continued axial pressure by the flex drive overcomes the compression spring force, and pushes the collet assembly out of the joint body and the collet fingers are extended into the receptacle. Rotating the flex drive at this point begins to advance the expander rod so that it forces the collet fingers outward. This joint requires only a low input torque to achieve a 3,000 lb. preload as a result of the collet's wedging action combined with its threaded actuation.
U.S. Pat. No. 3,982,841 discloses a node/strut assembly joint and method of attachment. A "T" head is inserted into a rectangular slot in the node ball. A screw which is mounted in relation to the strut axis, is rotated to cause a nut to turn within the joint through a coupling pin. As the threaded nut turns within the joint, the bolt is drawn into the joint forming an attachment. The "T" head is not used to aid in aligning the connector. It is inserted axially and then must be turned 90.degree. about the strut axis in order to effect a connection. A tool must then be inserted into the nut and rotated about the strut axis in order to tighten it. This sequence of operations would be very difficult for a robotic device to perform.
U.S. Pat. No. 4,334,797 discloses a coupling element with an arched, wedged distal end which is easily guided into a coupling element. A threaded screw is inserted through the side of the receiving coupler and tightened to lock the elements together.
U.S. Pat. No. 4,465,115 discloses a hammer with a head having a means for accepting a nail through a side entrance to guide the nail to its destination. The nail is easily released after the hammer is used to start the nail.
U.S. Pat. No. 4,509,882 discloses a furniture joint fastener. Fastening two furniture elements involves aligning oval elements until one is seated within the other. A cross shaped receiver is then turned camming the two furniture elements together.
U.S. Pat. No. 4,579,474 discloses furniture assembly and illustrates a means of using a cammed element to draw together two elements arranged normal to one another. A wide opening is available allowing easy entry to the bolt mechanism.
U.S. Pat. No. 4,974,987 discloses a strut/node joint having a hooked wire within the strut which is coupled to a cammed bolt. The wire is inserted into the node receptacle and the bolt is turned camming the hook into the strut. This provides a secure attachment rapidly.
U.S. Pat. No. 5,007,762 discloses a strut/joint having a receptacle with a side entry means. This joint has been proposed for EVA activity. A spring-loaded bolt in the strut is used to lock the coupling following attachment of the joint to the receptacle. This patent discloses the use of side entry to begin a connection process. This joint is very unforgiving of certain misalignments prior to insertion due to the tight fit between end and cavity. It relies on this tight fit in order to provide a load path between the two joint halves after fastening, but it imparts relatively little preload to the connection, and preload is an essential element in maintaining linearity in an overall truss structure.
There has long been a need for a truss joint which is robot-friendly. There has long been a need for such a joint whose assembly can be automated and can be accomplished during EVA.